All of this.

UA is going to do what it thinks will make it most profitable, whether that helps a single individual customer like the OP or not is mostly irrelevant. The strategy of filling F is going to fall somewhere between selling every flight out and giving upgrades to ensure loyalty from repeat flyers. Where exactly that balance lies is going to be different by market, management philosophy and other factors. If the airline improves profitability while continually retaining their most loyal and regular customers, than they know they are on the right track. If they lost a higher number of premiers because of the upgrade situation, they are going to know they threw that balance off too much, but there are always going to be changes to best optimize overall profitability.

The point of upgrades isn't for UA to give free stuff to customers instead of having them pay for it. They are doing it at the level that they think is necessary to bring the best overall profitability to the organization. Sometimes, that means letting premiers use 'currency' to upgrade into premium cabin, and sometimes even doing it for free [domestically]. Sometimes, that is going to mean selling it to folks at purchase, checkin, or even boarding or between any of these events. They are a business.

Honestly, if you really want premium cabin, the only way to guarantee it is to buy a ticket into that cabin. Yes, there are certain situations where upgrade odds are better or worse than others, but really, it is always going to be a gamble.